How to make a humanoid move like a car while running?

So in this script, the Parent is the humanoid. I made this but doesn't get my desired result. I have no idea how to make the WalkSpeed reset after the player stops running or dies.

local dude = script.Parent

if dude.WalkSpeed == 0 then
    script.Disabled = true
    dude.WalkSpeed = 5
elseif dude.WalkSpeed > 4 then  
    script.Disabled = false
    for i = 5,600,1 do
        dude.WalkSpeed = i
        wait()
    end
end